1. Field of the Invention
The present invention relates to a method of improving the signal to noise ratio (S/N ratio) of a noisy signal. In particular, the invention relates to a method for improving the S/N ratio of a seismic signal obtained while drilling a well using the drill bit as the seismic source.
2. Description of the Related Art
The present invention finds particular applicability in the seismic-while-drilling (SWD) situation in which a rollercone drill bit acts as the source of seismic signals while drilling the formation and the signals propagate to the surface via the formation and/or drill string where they are detected with geophones. For an SWD survey, the signal of interest is the signal which travels directly from the bit (or other point of contact between the drill swing and the borehole) to the geophones. Unfortunately, the geophones will also detect noise emanating from the rig due to machinery associated with the drilling process such as mud pumps and diesel engines which couple to the drill swing via the mud hose and other connections and travels along the surface directly to the geophones. This latter noise is known as ground roll and causes particular problems in SWD measurements.
The currently proposed technique for reducing the effect of ground roll in SWD surveys is to provide an array of several geophones, typically arranged in a line pointing towards the major source of ground roll, usually the drilling rig itself. The signals recorded at each geophone are summed together in analogue form to produce a single array output. The array is typically arranged such that its length is at least one wavelength of the lowest frequency component of the ground roll and the spacing between geophones is sufficiently small as to prevent aliasing of the highest frequency component. The total length of the array must be sufficiently small that the seismic signal arrives at substantially the same time at each geophone. When the geophone signals are summed the ground roll is attenuated but the seismic signal is preserved.
An alternative method of reducing interfering signals is proposed in U.S. Pat. No. 4,556,962. In this method, which relates to conventional seismic surveying, it is attempted to reduce the ground roll signal from a surface seismic source by placing a sensor close to the source to detect the interfering signal which is then amplified or attenuated, delayed and summed with the signals from the geophone array which are cross correlated with the original source signal. It is suggested that an adaptive filter be used so as to modify the delayed signal to correspond more closely to that detected by the array. The technique attempts to exploit the relative difference in the ratio of signal amplitude to ground roll amplitude between the geophone array and the sensor positioned close to the source. However, since the ground roll is dispersive in nature, the signal measured close to the source may be substantially different from that received by the geophone array and the adaptive filter might not be able to deal with this.